Question 155357
Let x=number of kilograms of 20% tin alloy needed

Now we know that the amount of pure tin in the 20% alloy (0.20x) plus the amount of pure tin in the 70% alloy (0.70*80) has to equal the amount of pure tin in the final mixture (0.50(80+x)).  So, our equation to solve is:

0.20x+0.70*80=0.50(80+x) get rid of parens and simplify

0.20x+56=40+0.50x  subtract 0.20x and also 40 from each side

0.20x-0.20x+56-40=40-40+0.50x-0.20x  collect like terms

16=0.30x  divide each side by 0.30

x=53.33 kilograms -----------amount of 20% alloy needed

CK
53.33*0.20+0.70*80=0.50*133.33
10.67+56=66.67
66.67=66.67

Hope this helps---ptaylor